1. Field of the Invention
The present invention relates to an illumination device and a display apparatus including the same; and specifically to an illumination device provided on a rear side of a display panel and usable as a backlight of a display apparatus, and a display apparatus including such an illumination device.
2. Description of the Related Art
Liquid crystal display apparatuses are widely used for electronic office equipment, vehicle-mountable TVs, monitors of video cameras and the like owing to the features of being light weight, thin and low in power consumption. Unlike self-luminous display apparatuses including CRTs (cathode-ray tubes), PDPs (plasma display panels) and EL (electroluminescence) units, liquid crystal display apparatuses use a liquid crystal display device which is not self-luminous. Therefore, transmissive liquid crystal display apparatuses include a planar illumination device referred to as a backlight provided on a rear side of the liquid crystal device. The amount of illumination light from the backlight, which is to be transmitted through the liquid crystal display device, is controlled by the liquid crystal display device pixel by pixel, and thus images are displayed.
Backlights are roughly classified into “direct”-type backlights and “edge light”-type backlights. A direct-type backlight includes a plurality of rod-like light sources such as fluorescent tubes directly below the liquid crystal display device. An edge light-type backlight includes a light source located at an end of a lightguide plate, and the light from the light source is guided to the liquid crystal display device by the lightguide plate.
FIG. 12 schematically shows a general structure of a direct-type backlight. In FIG. 12, a direct-type backlight 40 is provided on a rear side of a transmissive liquid crystal display panel 48. The backlight 40 includes a plurality of fluorescent tubes 41 arranged at a prescribed interval, a case 43 for accommodating the plurality of fluorescent tubes 41, and an optical sheet (for example, a diffusion sheet or a prism sheet) 45 located between the plurality of fluorescent tubes 41 and the liquid crystal display panel 48. The direct-type backlight 40 allows the luminance to be easily controlled by adjusting the number of the fluorescent tubes 41 used as the light sources. (Hereinafter, the fluorescent tubes 41 will also be referred to as the “light sources 41”.) Therefore, the direct-type backlight 40 can easily realize high luminance and so are often used for large-sized liquid crystal display apparatuses.
However, the direct-type backlight 40 has the following drawback. The light sources 41 exist discretely, i.e., at an interval directly, below the liquid crystal display panel 48. Therefore, the luminance is higher in areas of the liquid crystal display panel 48 right above the light sources 41 and is lower in areas of the liquid crystal display panel 48 corresponding to regions between the light sources 41. This results in uneven luminance.
The degree of unevenness of luminance is lower as distance A between the light sources 41 and the liquid crystal display panel 48 is longer, and also as distance B between two adjacent light sources 41 is shorter. In other words, the degree of unevenness of luminance is lower as the ratio R of distance B with respect to distance A (i.e., B/A) is smaller, and is higher as the ratio R is larger. This means that the degree of unevenness of luminance can be decreased by increasing distance A between the light sources 41 and the liquid crystal display panel 48, or by increasing the number of the light sources 41 and thus decreasing distance B between two adjacent light sources 41. However, these solutions also have the following problems.
In the case where distance A between the light sources 41 and the liquid crystal display panel 48 is increased, the thickness of the backlight 40 and so the thickness of the display apparatus including the backlight 40 are increased. This lowers the product value of the backlight 40 and the display apparatus. In the case where the number of the light sources 41 is increased so as to decrease distance B between two adjacent light sources 41, the production cost of the backlight 40 and the display apparatus including the backlight 40 is raised. This also lowers the product value of the backlight 40 and the display apparatus.
Japanese Laid-Open Patent Publication No. 2002-122863 discloses a backlight 50 shown in FIG. 13. The backlight 50 includes light-reflective projections 52 having a triangular cross-section, which are provided between light sources 51. With such a structure, the backlight 50 suppresses the unevenness of luminance.
Japanese Laid-Open Patent Publication No. 2000-310776 discloses a backlight 60 shown in FIG. 14. The backlight 60 includes an assisting light source 64 including a primary light source 62 and a lightguide plate 63. The assisting light source 64 is provided on the opposite side from the liquid crystal display panel with respect to the light sources 61. With such a structure, the backlight 60 suppresses the unevenness of luminance. Japanese Laid-Open Patent Publication No. 2000-310776 also describes that the unevenness of luminance can be further suppressed by forming a scattering dot pattern as follows. The scattering dot pattern is formed on a surface of the lightguide plate 63 in order to guide outside the light which is emitted from the primary light source 62 and incident on the lightguide plate 63. The dots are formed sparsely in areas right below the light sources 61 and densely in areas corresponding to regions between the light sources 61. With this structure, the luminance of the light going out from the lightguide plate 63 can be controlled such that the luminance of the light from the areas right below the light sources 61 is lower and the luminance of the light from the areas corresponding to the regions between the light sources 61 is higher. Thus, the unevenness of luminance can be further suppressed.
The present inventor studied in detail the relationship between the structure of the backlight and the degree of unevenness of luminance from various perspectives. As a result, it was found that the unevenness of luminance is not sufficiently suppressed either by the backlight disclosed in Japanese Laid-Open Patent Publication No. 2002-122863 or by the backlight disclosed in Japanese Laid-Open Patent Publication No. 2000-310776. More specifically, with the backlights disclosed in Japanese Laid-Open Patent Publications Nos. 2002-122863 and 2000-310776, the degree of unevenness of luminance is sufficiently decreased in the “front” direction (i.e., the direction normal to the display screen of the display apparatus) but is not sufficiently decreased in “oblique” directions (i.e., directions angled with respect to the direction normal to the display screen of the display apparatus).